The present invention relates to microwave furnaces and more particularly to a microwave furnace design that provides uniform power distribution within the furnace cavity.
When microwaves are applied into a closed space, such as a home microwave oven, a number of regular power distributions are created by the formation of standing waves having power peaks and valleys, with the valleys having little or no power intensity and the peaks having maximum power intensity. These distributions or modes overlap with one another in time effectively smearing out the power distribution. In order to achieve power uniformity in a "regular cavity", i.e. one with a conventional geometric shape, such as a cylinder or rectangular box, approximately one hundred modes need to be developed within the cavity. The number of modes are roughly governed by the equation:
M=L/.lambda. where M is the number of modes, L is the characteristic dimension of the cavity and .lambda. is the wave length of the radiation.
For a home microwave oven with a wave length of ten cm. (2.4 GHz.) to achieve one hundred modes, the characteristic box dimension L would need to be on the order of ten meters. Obviously, a home microwave oven is not quite this big. To achieve a regular cavity in this case, a "mode mixer" in the form of a fan acts to chop up the incoming microwaves. The resulting random distribution of modes helps to achieve a more regular distribution of energy.
The degree of regularity necessity for a home oven is not very great because of the nature of the materials being heated. The mechanical and thermal properties of food and the cooking temperatures of generally less than two hundred degrees centigrade make the occurrence of short-lived "hot spots" less critical. However, for industrial applications, such as the sintering of ceramic materials at temperatures greater than five hundred degrees centigrade, a thermal gradient within the sintering body of less than fifty degrees centigrade could be enough to induce cracking due to uneven thermal expansion. Additionally, when ceramic materials get hotter, their absorption of microwaves increases non-linearly. This can lead to a condition called thermal runaway, where the development of a hot spot can be self-propagating and cause localized melting, unequal densification during sintering, and poor quality sintered bodies.
Recently it has been found that ergodic modes can be utilized in a microwave oven rather than regular modes. Ergodic modes, while temporally coherent are not spatially coherent. Thus ergodic modes will not have a simple power distribution in space. Rather than appearing as regularly spaced regions of high and low power, ergodic modes will be essentially randomly spread around the cavity. This distribution of intensity for each ergodic mode means that fewer modes must overlap in order to obtain a regular power distribution within the cavity.
While regular modes will occur for simple shapes such as cubes, cylinders or spheres, the optimal ergodic modes will occur for a shape such as a "stadium". Shapes of this type will produce a uniform power distribution for values of M of only ten which means the value of L can be reduced to practical dimensions.
It is an object of the present invention to provide a microwave furnace design that induces the occurrence of ergodic modes so as to allow for an industrial microwave furnace having a practical characteristic dimension.